Piling it On

On the coast, the safest homes stand above the rest — on pilings

On the coast, the safest homes stand above the rest — on
pilings

by Clayton DeKorne

Decades of storm-damage assessments clearly demonstrate the need
for more stringent foundation requirements in Coastal A
zones

(Photo by Vince Lupo)

Along the eastern seaboard and Gulf Coast, most building codes
require beachfront homes to be elevated on piling with little or no
obstruction between vertical supports. In a severe storm, an
elevated, "open" foundation will allow floodwaters to wash
underneath the first floor without putting an excessive load on the
structure and bringing down the house. Under current National Flood
Insurance Program (NFIP) standards, which largely dictate local
building codes in coastal communities, open foundations are
required for homes in coastal flood zones that are rated for
"velocity wave action" (V zones). This typically includes only the
first row of homes on the beach. For homes in less exposed zones,
piling foundations are widely recommended but rarely required,
despite an overwhelming body of evidence that broader application
of V-zone standards would mitigate much of the damage from storm
surge.

SOURCE OF MAXIMUM DESTRUCTION

Storm surge is a swelling of ocean levels preceding a severe storm.
The swirling winds created by hurricanes and other cyclones can
push up a dome of water as large as 50 to 100 miles wide in front
of the storm, leading to an increase in water levels ranging from 4
to 6 feet in a minor hurricane, to greater than 20 feet in big
storms (when it combines with high tide). The level of the surge
depends, in part, on the slope of the continental shelf along the
coast. The shallower the coastal waters, the more floodwaters surge
ashore. In most places, the shelf surrounding the Atlantic and Gulf
shores is relatively wide and shallow, and most of this densely
populated coastline lies less than 10 feet above mean sea level.
Consequently, the danger from storm surge is tremendous. According
to the Federal Emergency Management Agency (FEMA), in nearly every
hurricane on record, storm surge is responsible for more deaths and
more property damage than any other effect of the storm, even high
winds.

In North Carolina, a 15-foot surge in front of Hurricane Floyd
flattened Oak Island's frontal sand dune, then took down homes like
the one shown on the left, which once stood on a ground-level block
foundation. Damage inspectors reported that the home likely would
have been saved if it had been elevated on piling like that shown
on the right. (Photo courtesy Dave Gatley/FEMA)

Waves can exert enormous pressures on buildings. While winds wield
pressures in the tens of pounds per square foot, a 2- to 3-foot
wave can exert pressures in the hundreds or even thousands of
pounds per square foot, says Christopher Jones, an engineer from
Durham, N.C., who wrote the current edition of FEMA's Coastal
Construction Manual-FEMA 55 (available free from FEMA;
800/480-2520). These are forces that ordinary solid walls can't
easily resist.

Despite the magnitude of the force, protection proves relatively
straightforward: Simply elevate the house above the surge. In storm
after storm, inspectors assessing damages report that homes
elevated on piling survive relatively unscathed compared to older
homes supported by solid foundations (see "Lessons Learned After
the Storm," below). Homes that are not elevated are frequently
inundated with floodwater or swept off their foundations
entirely.

LESSONS LEARNED AFTER THE STORM

Reports from FEMA inspectors assessing the damage to coastal
communities after major storms build a convincing case for broader
application of V-zone requirements in Coastal A zones. The
following excerpts relating to foundations are extracted from the
"historical perspective" presented in the Coastal Construction
Manual:

Hurricane Frederic, Alabama and Mississippi, 1979:
"Approximately 73 percent of front row homes were destroyed, while
only 34 percent of second and third-row buildings. The destruction
of non-elevated buildings was predictable; however large numbers of
elevated houses built to the BFE enforced at that time were also
destroyed. Analyses confirmed that much of the damage to houses
elevated to the BFE occurred because the BFE was based on the
stillwater level only. It was after Hurricane Frederic that FEMA
began to include wave heights in its determination of BFEs in
coastal flood hazard areas."

Hurricane Gloria, New York, 1985: "Oceanfront
homes had been left vulnerable to flood, erosion, and wave damage
by previous northeast storms. Accordingly, damage from Gloria
included settlement of inadequately embedded pilings, loss of
poorly connected beams and joists, failure of septic systems due to
erosion, and water and overwash to non-elevated buildings."

Hurricane Bob, Buzzards Bay Area, Massachusetts,
1991: "Buildings constructed before the date of the Flood Insurance
Rate Map (FIRM) for each community — referred to as pre-FIRM
buildings — that had not been elevated, or had not been
elevated sufficiently, suffered major damage or complete
destruction.... Post-FIRM buildings and pre-FIRM buildings with
sufficient elevation performed well during the storm."

Hurricane Hugo, S.C., 1989: "Post-FIRM buildings
that were both properly constructed and elevated survived the
storm. These buildings stood out in sharp contrast to post-FIRM
buildings that were poorly designed or constructed."

Northeaster, Nags Head, N.C., Kill Devil Hills,
N.C., and Sandbridge Beach, Va., 1989: "Failure of the pile-to-beam
connection was observed where a bolt head lacked a washer and
pulled through the beam.... Cracks in, or failed connection to,
piles and deck posts were, in some cases, attributed to
cross-bracing oriented parallel to the shore or the attachment of
closely spaced horizontal planks."

Hurricane Fran, southeastern N.C., 1996: "Many
buildings in mapped A zones were exposed to conditions associated
with V zones, which resulted in building damage and failure from
the effects of erosion, high-velocity flow, and waves.... Hundreds
of oceanfront homes were destroyed by the storm, mostly as a result
of insufficient pile embedment and wave effects."

This duplex built on a ground-level foundation prior to the
1986 North Carolina State Code was knocked apart by storm surge
from Hurricane Fran. (Photo courtesy Dave Gatley/FEMA)

A-ZONE RISKS

Post-storm assessments suggest the need for a broad redefinition of
design and construction standards in coastal A zones, but so far,
nothing public has been written into the NFIP. At issue is how
communities distinguish between places where homes must be elevated
and places where solid foundations are deemed safe.

Currently, the NFIP literally draws this distinction on Flood
Insurance Rate Maps (FIRMs) by specifying V zones and A zones. V
zones, which are the only zones designated high hazard areas,
include the shoreline subject to damage from waves 3 feet and
higher. In V zones, the lowest portion of the first-floor framing
must be elevated above the Base Flood Elevation, or BFE, and no
permanent enclosures are allowed below the first floor (see
"Deciphering Elevation Standards," below).

DECIPHERING ELEVATION STANDARDS

The flood zones (or Special Flood Hazard Areas) marked on flood
insurance rate maps, show locations with a 1 percent probability of
flooding to or above a specific elevation in any given year. For
each hazard zone on the map, FEMA specifies the likely flood
elevation, known as the Base Flood Elevation (BFE) — a height
determined by a statistical analysis of the last 100 years of
measured stillwater elevations and calculated heights for waves and
wave runup. Stillwater elevations (a misnomer) are the elevations
of the water surface resulting solely from storm surge, while wave
heights mark the crests of wind-driven waves above the surge
elevation. Wave runup is the rush of water up a slope or structure
caused by breaking waves.

The BFE marks the top of foundation piling in V zones under minimum
NFIP standards. However, a community may adopt a building code that
goes beyond this standard, identifying a Design Flood Elevation
(DFE) above the BFE. The height exceeding the BFE is called a
freeboard.

NFIP STANDARDS FOR PILING
FOUNDATIONAll first-floor framing must be
above the Base Flood Elevation, and pile embedments must account
for potential scour.

By contrast, A zones comprise areas subject to only general
flooding from the surge, but may include waves under 3 feet tall.
Under current NFIP standards, closed foundations are allowed in A
zones, as long as they include flood vents to equalize the pressure
of floodwaters, and the top of the first floor sits at or above the
base flood elevation.

The defining factor separating V zones and A zones is a 3-foot
breaking wave, which proves to be a rather arbitrary cut-off point.
Christopher Jones traces the 3-foot height standard to a study by
the Galveston District in 1975, which defined the "Critical Wave"
as "a wave possessing sufficient energy to cause major damage on
contact with conventional structures." An appendix in the study
concluded that this critical wave is a 3-foot breaking wave.
However, according to Jones, a closer reading of calculations in
the study shows that breaking waves 2.1 feet high are capable of
destroying conventional wood-frame walls and connections. In
addition, Jones reports, full-scale laboratory tests by FEMA
determined that breaking waves as low as 1 foot high consistently
cause failure of traditional stud-wall construction.

This analysis is consistent with actual storm-damage reports, as
well. Inspections following Hurricanes Hugo in South Carolina in
1989, Opal in Florida in 1995, and Fran in North Carolina in 1996,
all documented extensive wave and erosion damage in A-zone homes
that had been built in compliance with current A-zone standards.
These reports prompted investigation of the 3-foot-wave rule, which
led Jones and other floodplain managers to propose creating a
Coastal A zone — a flood zone beyond the V zone where
destructive waves less than 3 feet high are likely.

"Flood hazards in Coastal A zones are more like V zones than
riverine A zones," explains Christopher Jones. "Design and
construction requirements in coastal A zones should be more like
those in V zones, as well." Slowly, design and construction
requirements are catching on, but have yet to be fully integrated
into standards. FEMA's latest Coastal Construction Manual
identifies the Coastal A zone apart from V zone and ordinary A
zones. The manual "recommends" that foundations be built in Coastal
A zones just as those required in V zones. Namely, it stresses the
need to increase setbacks landward of the mean high-tide mark, and
the need to raise the first floor above the base flood elevation on
piling that is sized and spaced sufficiently to resist flotation,
collapse, and lateral movement.

Piling for decks and porches must be embedded to the same depth
as piling for the main structure, as shown on this home undergoing
renovation in Wrightsville Beach, N.C. Several severe storms in the
area caused excessive damage to buildings with decks or porches.
The flood forces took out shallow piles, leaving the houses
vulnerable to further damage once roof supports were gone and decks
were separated from the main structure. (Photo courtesy Dave
Saville/FEMA)

In addition, the American Society of Civil Engineer's national load
standard (ASCE 7) and flood-resistant design and construction
standard (ASCE 24) both specify V-zone load combinations in Coastal
A zones. Load combinations are an engineer's shorthand for
calculating a wide range of design loads — dead, wind, wave,
uplift, floatation, overturning, etc. All these loads must be
accounted for, but when they are tallied separately, they can lead
to an unnecessary over-design. The ASCE sets a reasonable standard
for combining loads without an excessive addition of them.
Currently, the ASCE standards are referenced in the International
Residential Code. However, in most cases they would influence the
structural design of a home in a Coastal A zone only if an engineer
were involved. So far, engineers are required to certify only
V-zone houses.

A few coastal communities have voluntarily adopted V-zone standards
in Coastal A zones — action that the NFIP rewards with lower
insurance premiums for property owners under its Community Rating
System (CRS). For example, the barrier island of Pensacola Beach in
the Florida Panhandle adopted V-zone standards for all
A-zone-mapped residential structures in Pensacola Beach in1987. The
ruling paid off in 1995 when Hurricane Opal brought a storm surge
that washed over the entire island. "We didn't lose any structures
that had been built to the code," reports Debbie Norton, manager of
the Santa Rosa Island Environmental and Developmental Services
Department, the code authority with jurisdiction over Pensacola
Beach. "Opal overwashed the entire island and took out [many] of
the older houses on solid foundations. But every new house on
piling survived."

Pensacola Beach remains an exception, at least for now. "Bottom
line," says Jones, "the widespread use of open foundations in
Coastal A zones will not occur until FEMA says, 'Do it.'"

MAXIMUM UNBRACED AND BRACED PILING
HEIGHTS: 8-IN. ROUND PILES IN SAND

One Story

Max. Unbraced Height (FT).

Max. Braced Height (FT).

Building
Dimension

Pile
Spacing

Dense
Sand

Loose
Sand

Dense
Sand

Loose
Sand

20'

10'-0"

8

9

12

13

22'

11'-0"

8

9

12

13

24'

12'-0"

8

9

12

13

24'

8'-0"

8

10

12

14

26'

8'-8"

8

10

12

14

28'

9'-4"

8

10

12

14

30'

10'-0"

8

10

12

14

32'

10'-8"

8

10

12

14

32'

8'-0"

9

10

13

14

34'

11'-4"

8

10

12

14

34'

8'-6"

9

10

13

14

36'

12'-0"

8

10

12

14

36'

9'-0"

9

10

13

14

38'

9'-6"

9

10

13

14

40'

10'-0"

9

10

13

14

40'

8'-0"

9

10

13

14

20'

10'-0"

6

8

10

12

Two Story

Max. Unbraced Height (FT).

Max. Braced Height (FT).

Building
Dimension

Pile
Spacing

Dense
Sand

Loose
Sand

Dense
Sand

Loose
Sand

22'

11'-0"

6

8

10

12

24'

12'-0"

6

8

10

12

24'

8'-0"

7

9

11

13

26'

8'-8"

7

9

11

13

28'

9'-4"

7

9

11

13

30'

10'-0"

7

9

11

13

32'

10'-8"

7

9

11

13

32'

8'-0"

7

9

11

13

34'

11'-4"

7

9

11

13

34'

8'-6"

7

9

11

13

36'

12'-0"

7

9

11

13

36'

9'-0"

7

9

11

13

38'

9'-6"

7

9

11

13

40'

10'-0"

7

9

11

13

40'

8'-0"

8

10

12

14

Notes:

1. Building Dimension is the length or width of the
building segment being designed. The design shall satisfy the
smaller unbraced or braced height required by the building width or
length.
2. Maximum Unbraced Height is the greatest distance above natural
grade for a pile without knee braces.
3. Maximum Braced Height is the greatest distance above natural
grade for a pile with knee braces.

Typical piling heights above grade (see chart) may be
compromised by erosion. The change in color on the support piers of
this house in Kitty Hawk, N.C. (above), shows the amount of beach
removed by the storm surge of Hurricane Isabel. (Photo courtesy
Mark Wolfe/FEMA)

BUILDING TO LAST

Jones and Norton stress that changing the code to account for
Coastal-A-zone hazards is just the first step. The quality of
construction ultimately matters the most. In particular, the depth
of embedment and the quality of connections between the piling and
the framing will ensure that a piling foundation can withstand the
impact of a storm. "Inspectors can only do so much," Norton
adds.

Embedment. Unlike a foundation that rests on a
footing, most driven piles support loads by friction along the
length of their sides. The embedment depth depends on the soil
characteristics, and most jurisdictions require an engineer's soil
analysis. A typical pile length for residential homes is between 20
and 60 feet.

The piling embedment specified by the engineer must account for
predicted scour, the erosion around a fixed object. If the soil
erodes around piles in a storm, the remaining embedment must still
be sufficient to resist uplift and provide lateral support.

Girders and joists. Ultimately, all loads must resolve to
the pilings by way of girders, blocking, and often, cross-bracing.
Because of the complex load paths involved and the likelihood that
the home will be tested by high winds, water, and impact from
waterborne debris, most jurisdictions require that the entire
structure be engineered.

In regions with prescriptive codes, girders supporting floors on
piling must be comprised of a minimum of two CCA-treated 2x12s
bolted together. Engineered beams, such as treated Parallam or
glulams, are often considered labor-saving alternatives. Splices in
beams are typically required over pilings.

Because of the difficulty of driving piling perfectly plumb, the
outer pilings of a foundation array are usually placed 6 to 10
inches in from the building line. This means the joists will
cantilever a short distance beyond the girders. As a rule of thumb,
the cantilever distance should never exceed the girder or joist
depth.

A home that was washed off its foundation by Hurricane Isabel
underscores the need for a strong connection between the
first-floor framing and the piling. (Photo courtesy Cynthia
Hunter/FEMA)

Since the girders are rarely on the building line and may be
slightly out of parallel, the trick is squaring the floor. A
preferred approach is to lightly tack the joists to the girders.
When all the joists are in place, the whole floor can be squared up
and then each joist fastened to the girders with hurricane ties,
such as the Simpson H6. It is easiest to install hurricane ties
before the subfloor is installed, though they can go in later. Use
only corrosion-resistant fasteners and hot-dipped galvanized nails,
and paint the edges of any custom fabricated hardware made from
galvanized steel with a cold galvanizing compound to stave off
corrosion.

Girder connections. When a piling foundation fails
in a storm, it's usually because the connection at the top of the
piling fails, which causes the house to wash off the foundation. A
strong connection between piling and girder requires a notch in the
piling and several galvanized 3/4- or 7/8-inch-diameter steel
bolts. Although it might seem easier to simply flat-cut the tops of
the pilings and use metal straps to secure the girders, this
entails the added cost of additional bolts and metal straps, plus
the added labor of drilling the extra bolt holes and cutting flat
sections on the sides of the pilings for the metal straps.

The notch for a piling-to- girder connection should not be
deeper than half the cross-section of the pile. The connections,
which must be specified by an engineer for V-zone foundations,
typically follow the bolting schedule shown at below. (Photo
courtesy JLC and Boardwalk Builders)

REQUIRED BOLTS FOR BEAM-TO-PILING
CONNECTION

Spaced-Beam Method

Double-Beam Method

Building
Type

Connection
with Splice

Connection
without Splice

Connection
with Splice

Connection
without Splice

One Story

2 bolts through each beam and pile (4 total)

4 bolts through beams and pile

2 bolts through each beam and pile and 2 bolts through plate
and pile (6 total)

3 bolts through beams and pile and 1 bolt through plate and
pile

Two Story

3 bolts through each beam and pile (6 total)

6 bolts through beams and pile

2 bolts through each beam and pile and 2 bolts through plate
and pile (6 total)

4 bolts through beams and pile and 2 bolts through plate and
pile

The most common mistake made is over-notching the pile. By code,
the depth of the notch should not exceed half the diameter of the
piling. The entire girder rarely needs to bear on the seat cut of
the notch. The engineer will often specify a bearing of about
one-third the diameter of the piling. However, occasionally there
are pilings where a deeper notch cannot be avoided — where
girders meet at corners, for instance. In this case, or when a
piling is so out of alignment that full bearing is not possible,
engineers may require a steel L-bracket to reinforce the
connection.

During Hurricane Floyd, the foundation of this Oak Island,
N.C., home failed because the homeowners nailed horizontal planks
to pilings to enclose a parking area, which increased surge forces
on the piling. Also, a section enclosed by breakaway walls had been
converted to living space, and the wiring added in the walls
prevented the walls from breaking away. (Photo courtesy Dave
Gatley/FEMA)

Piling does not have to fall perfectly on the building line.
Typically, floor joists cantilever slightly (no more than the
length of the joist depth) over the girders. The joists are
initially tacked in place so the whole deck can be squared up, and
then they are securely fastened to the girders with hurricane ties.
(Illustrations courtesy Texas Department of Insurance)

Bracing. Cross-bracing at the pilings may be
necessary to resist lateral loads from wind or water. However, the
cross-bracing may also catch debris and impede the flow of water,
increasing the load on the foundation. FEMA's Coastal
Construction Manual recommends using a larger pile and closer
spacing to allow for an increase in unbraced piling height, rather
than relying on an extensive amount of cross-bracing. If
cross-bracing is used, it's best to install it perpendicular to the
shoreline, so the bracing does not become a barrier to incoming
waves.

Grade-level enclosures. In V-zones, pilings can be
enclosed only by lattice, screen, or breakaway walls. While allowed
under NFIP standards, even the use of breakaway walls may be
illegal in some jurisdictions because they are difficult to monitor
after the house is occupied. Homeowners, anxious to maximize the
space of their high-priced properties, are often tempted to convert
enclosed areas to living space, and add wiring, plumbing,
countertops, closets, partition walls and built-ins — all of
which reduce the ability of these walls to break away easily in
surge conditions. Instead, the surge forces are transferred to the
structural piling, increasing the chances that the piling will
fail.

OWNER EDUCATION

Building on piling is no picnic. Drilling 1-inch-diameter holes
through 12 to 14 inches of piling for bolt connections 10 to 12
feet off the ground, and then cutting square notches in piling
that's anything but straight and plumb, will test the skills of the
very best builders. But in the end, the most difficult task for
contractors may be educating owners on the wisdom of increased
setbacks and more expensive engineered foundations, particularly in
Coastal A zones where they are not strictly required but where they
make perfect sense. "People tend to forget if they haven't lived
through a hurricane," explains Debbie Norton. "We don't get many
complaints anymore, not since Opal. But in places that have not
seen a major storm, tougher standards for Coastal A zones are a
hard sell."